TANIA AUSTRALIS (Mu Ursae Majoris). Our Ursa
Major, the Greater Bear, contains remnants of ancient Arabic
constellations, the best-known example the star Alkaid, which refers to the leader of the
daughters of a funeral bier. Southwest of the Dipper's bowl lie three obvious pairs of stars that
represent the bear's paws, but to the Arabs were the tracks of
leaping gazelles. The middle pair is the "second leap," from which
comes the name "Tania" (for "second"). In the multi-cultural mix
of constellation lore, the northern one received the Arab-Latin
name
Tania Borealis, the southern Tania Australis. Bayer assigned
the three "leaps" ordered Greek letters, Tania Borealis receiving
Lambda, Tania Australis Mu. The two make a lovely contrast, Tania
Borealis a white class A subgiant, Australis a fairly rare (for
naked eye stars) red class M (M0) giant. Tania Australis shines at
mid third magnitude (3.05) from a distance of 250 light years
(double the distance of Borealis, the two only a line-of-sight
coincidence). When we account for infrared radiation from a 3950
Kelvin surface, Tania Australis is found to have a luminosity 850
times that of the
Sun, which leads to a radius 62 times solar (0.28
Astronomical Units, three-fourths the size of Mercury's orbit).
Having used its core hydrogen, Tania Australis seems to be
brightening along the "red giant branch" with a contracting helium
core. Before long, the helium will fire up to fuse to carbon, and
the star will dim some and stabilize as a class K giant. Tania
Australis is an unresolvable binary, the companion (known only from
spectroscopic observations) circling the M giant every 230 days at
a distance of at least 1.5 Astronomical Units, suggesting a
combined mass over 9 times solar, double that expected on the basis
of luminosity and temperature. Tania Australis proper (ignoring
the companion) is also a rare "hybrid star." Magnetically active
stars like the Sun blow a relatively fast but thin wind from their
surfaces. Larger giants blow slower, but much thicker winds.
Hybrid stars seem to blow both. Though the star is cooler than the
dividing line at which stars seem to lose their X-rays, Tania
Australis perversely still seems to radiate them. Alas, the
southern star of the second leap is not very well understood.